In an optical communication system, optical signals may be transmitted in free space, but are generally transmitted over optical waveguides, typically optical fibers. Since optical fiber transmission offers tremendous bandwidth and transmission rate advances over the transmission of electrical signals, conversion to electrical signals are avoided as much as possible by active optical processing such as optical amplification, switching and routing. It is usually desirable to avoid conversion of the signals to electrical signal until they reach the target destination, where they are converted back to electrical signals representing digital data, voice or images in various analog formats.
In order to maximize the capacity of fiber optic communication systems many signal are simultaneously transmitted over the same fiber waveguides in a scheme known as wavelength division multiplexing or WDM. In WDM each discrete signal may correspond to a different wavelength of light, known as an optical channel. Various non-linear properties of optical glass, active and passive components in the optical system produce cross talk between the WDM optical signal channels. This xe2x80x9ccross talkxe2x80x9d is insignificant if the signal to noise ratio is high and the power levels of all optical channels are comparable.
The optical devices and interconnections in any route will result in signal losses, thus the signal power and signal to noise ratio of any optical signal can be expected to vary with the routing path. When the communication system is a network, optical channels are combined and routed together in common waveguides with signals from different sources, the power levels in each optical channels are likely to be different, in which case the xe2x80x9ccross-talkxe2x80x9d from the stronger channels will degrade the signal to noise ration in the weaker channels.
Degradation is avoiding by balancing signal power, such as by selectively attenuating the stronger signals. The level of attenuation may need to be varied as a network is modified, expanded or actively modified so as to accommodate the variation in communication traffic that can be expected with customer use and demand.
Thin film optical filters may be used to attenuate such optical signals. A thin film filter comprises one or more layers of material, each layer pre-selected for its optical properties, at a specific thickness to provide spectral control by interference phenomena arising from the interaction of light reflecting off the multiple thin film layers and/or absorption of energy by the thin film material.
Optical filters which utilize the inherent optical absorption of thin film materials are simpler in construction and avoid the characteristic reflected signal of an interference based filter, which must be managed to avoid cross talk or feedback to semiconductor lasers in the optical system. Although interference filter require more layers, they can be fabricated from dielectric materials having high stability to both environmental cycling of temperature and humidity as well as exposure to high optical powers.
Since an optical communications system must be highly reliable, yet adapted to variations in customer demand and utilization patterns, the ideal attenuation filter provides a reliably stable level of attenuation, preferably with a minimum attenuation variation, over all the signal channels in the WDM system.
Accordingly, it is an object of the present invention to provide an attenuating filter for optical communications systems that is stable over many years of use and provides uniform attenuation over a very broad wavelength range in a WDM system.
It is an additional object of the present invention to provide an attenuating filter and a method of adjusting the attenuation levels without sacrificing the uniform attenuation or low reflectivity a very broad wavelength range in a WDM system.
It is an additional object of the present invention to provide an attenuating filter and a method of adjusting the attenuation levels without sacrificing the uniform attenuation or low reflectivity a very broad wavelength range in a WDM system.
The attenuating filter provides both low reflectivity and uniform attenuation over the telecommunications C and L bands. The filter comprises a substrate, an attenuating coating deposited on at least a portion of the first surface of the substrate, a buffer coating deposited after the attenuating coating and has additional thin film layers to protect the attenuating coating from the environment and act as an antireflection coating on the clear locations on the substrate.
The attenuating filter preferably has a second antireflection coating deposited on the second surface of the substrate such that the reflectivity on the second surface of the attenuating filter is less than about 0.1 percent.
We have discovered that the appropriate selection of buffer layer and its incorporation within the antireflection coating design provides both the desired spectral control characteristics, but also has enhanced stability and resistance to degradation at high levels of incident light utilized in an optical telecommunications system.
These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims.